Reinforced composite pipe construction

ABSTRACT

A pipe and method of manufacture of the pipe are described. The pipe comprises a core of helically wound steel strip embedded in a plastics material matrix and lined with inner and outer linings of filament wound fiber reinforced plastics material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a composite construction of pipe foruse in the oil, gas, water and chemical industries.

2. Discussion of Prior Art

It is known to manufacture pipe by the helical winding of a plurality ofmetal strips and which strips are embedded in a plastics materialmatrix.

U.S. Pat. No. 4,657,049 describes the manufacture of such pipe byhelically winding at least one metallic reinforcing strip onto amandrel, the strip being coated with and embedded within a polymericbonding material. The tube is thus formed of a plurality of successivehelical convolutions of metal strip, completely embedded in thepolymeric material. In this construction the polymeric material providesonly a limited degree of corrosion resistance and hence is not suitablefor many pipe applications.

U.S. Pat. No. 4,351,364 describes a similar method of tube construction,but the tube also has inner and outer linings of resin impregnatedglass-fibre layers. The glass-fibre layers comprise woven cloth andchopped fibre strand mat. The purpose of the inner lining is to providecorrosion resistance and a low flow resistance, whilst the purpose ofthe outer lining is to provide resistance to environmental conditions.Whilst the glass-fibre reinforced linings on the inside and outside ofthe pipe enhance the resistance to corrosion, abrasion and otherenvironmental conditions, they do not significantly improve the strengthof the pipe, nor do they provide optimum corrosion resistance owing tothe porosity which is inevitably retained within the glass fibre mat andcloth.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a pipe having aconstruction which endows it not only with corrosion and abrasionresistance, but also with enhanced strength and stiffness. It is afurther object to provide a pipe construction which when used in highpressure applications will produce a leak failure mode at elevatedpressures close to ultimate burst pressure.

According to a first aspect of the present invention there is provided apipe having inner and outer linings of fibre reinforced plasticsmaterial, the inner and outer linings having therebetween a core ofhelically wound steel strip, the steel strip being embedded in aplastics material matrix wherein the inner and outer linings arefilament windings.

The filament windings may be mono-filaments or may be wound frommulti-filament tows.

The filament windings may be laid at an angle of + and -55° relative tothe pipe axis, with an error margin of + or -5°. A winding angle of +and -55° is chosen where it is desired to achieve optimum balance ofhoop and axial loads, such as in the case where a cylindrical pipe hasto withstand full pressure and loads. The precise angle is chosen so asto suit the operating requirements of the pipe.

Alternative layers or groups of layers of filament windings may be laidat different positive and negative angles with respect to the pipe axisand to each other.

It has been found that the use of filament wound fibre-reinforced innerand outer linings not only further enhances the resistance to corrosion,abrasion and environmental attack, but also imparts additional strengthand stiffness to the complete pipe. The degree of additional strengthand stiffness may be determined by calculation, but will depend upon thefundamental lining and strip material properties, the ratio of lining tototal steel thickness and the helical angle of lining windings to thepipe axis. Because of the additional strength imparted by the filamentwound linings, it has been found that the number of steel layers may bereduced to produce a pipe of a given strength, thus making the pipe moreeconomic to produce and lighter in construction. Furthermore, the use offilament winding is capable of producing a higher integrity, lowerporosity resin matrix than corresponding prior art constructions. Priorart tube constructions employing glass-fibre reinforced plastics (GRP)are unsuitable for high pressure applications due to micro-cracking andporosity as a result of the difficulty in fully impregnating woven andchopped strand type fibre reinforcements. The pipe of the presentinvention overcomes these problems because of the substantially improvedimpregnation level of the filament wound linings. The reduced porosityin the lining of the pipe of the present invention has the advantage ofsignificantly reducing the possibility of corrosive attack of theembedded steel strip.

The use of filament wound linings also allows the pipe of the presentinvention to be used in high pressure applications. The porosity in theprior an constructions lead to local strains under pressure giving riseto local cracking, even at relatively low pressures, causing eventualleakage and failure.

At operating pressures, the filament wound linings are subjected only tolow strain levels due to the presence of the relatively higher modulusstrip layers. The pipe pressure containment capability may be maximisedby ensuring that the strain capability of the linings is such that theyfail at a predetermined pressure only after the steel layers have beenloaded to a significant proportion of their ultimate strain, preferablygreater than their yield strain. An effective means for determining thestrain capability of the linings is by appropriate selection of thewinding angle of the filaments. A significant advantage of the presentinvention is that the strain capability of the pipe construction may becontrolled so as to give a leak failure mode at a predetermined pressurebefore ultimate pressure bursting occurs.

In this connection we have found that the contribution of the inner andouter filament wound linings to the ultimate pressure containmentcapacity of the pipe becomes significant at pressures above whichyielding of the steel strip has taken place. It is advantageoustherefore for the pipe construction to be so designed that leakage ofthe liners under internal pressures occurs at a hoop strain level atwhich the steel strip is in a yielded condition though not liable toultimate failure.

This is illustrated in FIG. 1 of the accompanying drawings, which showsa graph comparing typical internal pressure versus hoop straincharacteristics for (a) a pipe construction in accordance with thepresent invention comprising a helically wound steel core with internaland external filament wound fibre reinforced plastics linings; (b) thecore without the linings; and (c) the linings without the core.

It will be seen from FIG. 1 that by ensuring that the lining strainfailure occurs at around 1% ie. beyond the yield point of the steel core(0.5% strain) but at a strain less than the ultimate steel failurestrain, (3%) a leak before burst failure mode is ensured, and at anelevated pressure, approaching, but always below, the ultimate burstpressure of the pipe.

For applications with fluids with which the plastics lining material isincompatible, the pipe of the present invention may also be providedwith an impermeable lining such as aluminium, thermosetting plasticsmaterial or silicone rubber, for example, on which the inner filamentwound lining is formed. The addition of an impermeable lining ormembrane may alter the leak failure mode unless the strain to failurecharacteristics of the membrane are carefully matched to those of thepipe construction.

The material of the filament windings may be glass-fibre for mostapplications. However, other continuous fibre materials such as aramidfibre, eg Kevlar (Trade Mark), or carbon fibre may be used incombination with or instead of glass-fibre. The type of fibre used willdepend upon the application in which the pipe is to be employed. In someapplications, more than one type of fibre may be employed in a singlepipe.

The thickness of the inner and outer linings will vary, depending uponthe specific requirements for the pipe. However, in the case of glassfilaments, the inner lining will generally be of a minimum thickness of2 mm whilst the outer lining will generally be of a minimum thickness of1 mm.

The steel strip core may be formed of a plurality of layers of helicallywound strips which abut along their edges. Alternatively, the steel coremay be wound from one or more steel strips wherein each succeeding turnoverlaps the previous turn in the axial and radial directions.

Ideally, in the case of strips which abut along their edges at any givensection through the pipe wall, the winding pattern of the helicallydisposed steel strips is so arranged that no axial gap between adjacentedges of the wound strip in any layer coincides in the radial directionwith the axial gap of any other layer. Therefore, there is no right linepath with respect to the tube axis from the inside to the outside of thetube which passes through more than one of the axial helical gaps. Inthis way, there is no position in the pipe at which the effective steelthickness is reduced by more than one strip thickness of steel. Forlarge numbers of steel strip layers, the winding pattern may be repeatedso that in fact there may be coincidence of the axial gaps of two ormore layers with only modest reduction in axial strength.

For small helical gaps the local reduction in steel thickness onlyaffects the tube axial strength such that for pipes having three or morelayers of steel, the overall pressure vessel strength is substantiallyunaffected. By minimising the axial gap, within manufacturinglimitations, the hoop strength under internal pressurisation isunaffected, and as stated above, the axial strength is only reduced tothe extent of one steel strip thickness for each coincident axial gap.Therefore the steel core of a tube having eight layers of steel has ahoop strength equivalent to the full eight layers and an axial strengthequivalent to seven layers. When applied to the loading conditions of aclosed vessel internally pressurised, it may be seen that as hooploading is twice axial loading, the effective reduction in thickness inthe axial direction does not detract at all from the ultimate pressurestrength of the pipe. In terms of stiffness, the effect of the axial gapbetween the steel strip adjacent edges is the same in both the hoop andaxial directions. Therefore, the steel strip layers can quite accuratelybe considered as an isotropic material in which the effective modulus ofelasticity is simply reduced by an amount approximately equal to theratio of the width of the resin gap to that of the steel strip. Ofcourse, for pipes constructed in accordance with the present invention,there will be a significant additional contribution to both the axialand hoop strength of the pipe from the filament wound inner and outerlinings.

Pipes of any diameter may be produced, but typically they will lie inthe range from about 150 mm to about 1000 mm. The maximum stripthickness and width is determined by the mechanical requirements forwinding, ie required pipe diameter to strip stiffness and helicalwinding angle. The strip width is also partially governed by the need tomaintain an overlap pattern such that there is minimum coincidence ofaxial strip edge gaps from the inside to the outside of the pipe, andsuch that the axial interlaminar shear loads do not cause failure. Inpractice, for pipes having diameters between 150 mm and 1000 mm, thesteel strip may have a width lying in the range from 50 mm to 250 mm,and a thickness lying in the range from 0.12 mm to 1 mm.

The axial gap between adjacent strip edges of successive helical turnswill preferably be no more than 5 mm, and may generally lie in the rangefrom 1 mm to 3 mm.

The steel is prepared prior to winding to provide a suitably cleansurface for bonding with the resin. Suitable preparation techniquesinclude grit blasting and/or various known methods of chemical cleaning.

The resin may be an epoxy resin or any type suitable for the intendedapplication of the pipe.

The resin may also include fillers.

According to a second aspect of the present invention there is provideda method of making a pipe, the method comprising the steps of forming aninner lining by filament winding of a fibre material onto a mandrel at apredetermined angle with respect to the pipe axis, providing a resinmatrix for the fibre layer, helically winding a steel strip core ontosaid fibre material layer, providing said steel strip core with a resinmatrix, forming an outer lining by filament winding of a fibre materialonto the outside of the pipe at a predetermined angle with respect tothe pipe axis, providing said outer layer of fibre material with a resinmatrix, at least partially curing said resin matrix and removing saidpipe from said mandrel.

Preferably, the steel strip core may have at least three layers ofhelically wound steel strip.

The inner and outer linings may be formed by passing glass or otherfilaments through a resin bath prior to winding onto the mandrel oroutside of the tube which may be rotated by suitable drive means.

The method may further include the step of providing an initialnon-filament wound, fibrous layer on the mandrel which is also providedwith a resin matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more fully understood, anexample will now be described by way of illustration only with referenceto of the accompanying drawings, of which:--

FIG. 1 shows a graph comparing internal pressure versus hoop straincharacteristics for various pipe constructions;

FIG. 2 shows a schematic axial cross section through a tube according tothe present invention; and

FIG. 3 which shows a schematic representation of the manufacture of thetube of FIG. 2.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

Referring now to FIGS. 2 and 3 of the drawings where the same featureshave common reference numerals. A length of pipe indicated generally at10 is produced by winding a plurality of layers onto a heated rotatingmandrel 12. The mandrel is coated with a known release agent (notshown).

A resin rich inner surface 14 is provided by helically wrapping a "C"glass or polyester veil onto the mandrel and impregnating with asuitable epoxy resin such as MY 750 supplied by the Ciba-Geigy companywith a suitable hardener system. Successive helical turns of the veilmaterial are overlapped in the axial direction, the width of the veilmaterial being about 150 mm.

A thickness of GRP inner lining 16 is then built up by helicallyapplying filament windings from a creel 17 of "E" glass fibre filaments18 laid at 55° to the tube axis. The filaments 18 are passed through aresin bath (not shown) immediately prior to winding so that the windingsare effectively provided with a resin matrix as they are being wound. Anumber of filament rovings are laid such that the minimum thickness isabout 2 mm.

Prepared steel strip 20 from coils 22 is then helically wound onto thestill uncured wet resin of the inner lining 16, successive helical turnslying adjacent each other with a maximum axial gap 24 of 5 mm. An epoxyresin 26 containing an appropriate filler, is simultaneously applied byknown means to the steel strip as it is wound onto the pipe such that inthe finished pipe, each layer of steel is coated with and bonded to thenext layer by the resin system. The number steel layers and the totalthickness is determined by the required pressure, stiffness rating anddiameter of the pipe and the combined mechanical properties of the steeland linings. The helical windings of the steel strips are axially offsetto each other so that there is no right line path from the inside to theoutside of the tube through more than one of the axial gaps 26, and asindicated by the dashed line 28.

An outer lining 30 is applied by a number of rovings of 55° helicalwindings of an "E" glass fibre filament 32 from a creel 34. As with theinner lining, the filament 32 is passed through a resin bath (not shown)immediately prior to winding. The thickness of the outer lining 30 is aminimum of 1 mm.

Whilst still rotating, heat is applied to the thus constructed pipe toraise the temperature for the minimum time which will satisfactorilysolidify or cause the resin to gel. The mandrel with gelled or curedpipe assembly is removed from the winding machine (not shown) andallowed to cool to ambient temperature whereupon the mandrel is removedfrom the pipe. The pipe may then be post-cured in a free standingposition if required. After cooling to ambient temperature, the pipe iscut to finished length by grinding through the total wall thickness, ieGRP and steel, to remove a minimum of two pitch lengths of steel fromeach end.

Although the method described above involves heating of the mandrel 12during the winding process, this may be unnecessary with certain typesof resin materials, particularly those having a short curing time.

We claim:
 1. A composite pipe, said pipe comprised of:inner and outerlinings of fiber reinforced plastics material, and a core of helicallywound steel strip located between said inner and outer linings, thesteel strip is embedded in a plastics material matrix, said inner andouter linings are filament windings, each of said inner and outerlinings comprising a plurality of layers of continuous fiber embedded ina plastics matrix.
 2. A pipe according to claim 1 wherein the filamentwindings are monofilaments.
 3. A pipe according to claim 1 wherein thefilament windings are multi-filament tows.
 4. A pipe according to claim1 wherein alternate layers of filament windings are laid at differentpositive and negative angles with respect to the pipe axis.
 5. A pipeaccording to claim 1 wherein alternate layers of filament windings arelaid at different positive and negative angles with respect to eachother.
 6. A pipe according to claim 1 wherein the filament windings arelaid at an angle of + and -55° relative to the pipe axis.
 7. A pipeaccording to claim 6 wherein the angle of the filament windings have anerror margin of + or -5°.
 8. A pipe according to claim 1 furtherincluding an impermeable lining inside the filament wound inner lining.9. A pipe according to claim 8 wherein the impermeable lining isselected from a material in the group comprising aluminium,thermosetting plastics materials and silicone rubber.
 10. A pipeaccording to claim 1 wherein the filament wound lining material is atleast one selected from the group comprising glass fiber, aramid fiberand carbon fiber.
 11. A pipe according to claim 1 wherein the thicknessof the inner filament wound lining is a minimum of 2 mm.
 12. A pipeaccording to claim 1 wherein the thickness of the outer filament woundlining is a minimum of 1 mm.
 13. A pipe according to claim 1 wherein thecore of helically wound steel strip comprises a plurality of layers ofhelically wound strips.
 14. A pipe according to claim 1 wherein the coreof helically would steel strip comprises at least three layers of steelstrips.
 15. A pipe according to claim 1 wherein the steel core comprisesa plurality of layers of strips defining a predetermined windingpattern, said winding pattern of the steel strips provides no axial gapbetween adjacent edges of the wound steel strip in any one layercoincides in the radial direction with the axial gap between adjacentedges of the wound steel strip of any other layer in said windingpattern.
 16. A pipe according to claim 15 wherein the winding pattern ofsteel strips is repeated in the radial direction.
 17. A pipe accordingto claim 1 wherein there is a gap between adjacent edges of successiveturns of said helically wound steel strip, said gap being a maximum of 5mm.
 18. A pipe according to claim 17 wherein the gap between adjacentedges of successive turns lies in the range from 1 mm to 3 mm.
 19. Apipe according to claim 1 wherein the plastics material matrix is anepoxy resin.
 20. A pipe according to claim 1 wherein the plasticsmaterial resin also includes a filler.
 21. A pipe according to claim 1wherein the steel strip of the core has a width lying in the range from50 mm to 250 mm.
 22. A pipe according to claim 1 wherein the steel stripof the core has a thickness lying in the range from 0.12 mm to 1 mm. 23.A pipe according to claim 1 wherein it has a diameter lying in the rangefrom about 150 mm to about 1000 mm.
 24. A pipe according to claim 1,wherein the strain capability of the linings in response to increasinginternal pressure within the pipe is such that the linings fail, andtherefore provide a leak failure mode, only after the steel core hasbeen loaded to a significant proportion of its ultimate strain.
 25. Apipe according to claim 24, wherein the pressure strain capability ofthe linings is such that they provide a leak failure mode at apredetermined internal pressure after the steel core has been loadedbeyond its yield strain.